Optical communication systems generally employ an optical source, typically a semiconductor laser, for generating an optical carrier signal and a modulation scheme for impressing information on the carrier signal. In one modulation scheme, known as direct modulation, some parameter of the laser is directly altered in accordance with the information to be transmitted. Commonly, the drive current of the laser is modulated so that the output of the laser is pulsed. In other schemes, the laser generates a continuous wave (CW) carrier signal, and an external modulator alters a parameter of the carrier signal (e.g., its amplitude, frequency, or phase). The latter arrangements are becoming more and more of interest in high speed digital systems where direct modulation at bit rates in the tens of giga-hertz range is beyond the capability of present semiconductor lasers. However, external modulators are also utilized in analog systems, such as CATV systems, where they produce lower frequency chirp than directly modulated semiconductor lasers. FIG. 2 illustrates a typical optical transmitter 20 for use in such systems. A laser source 12 generates a CW carrier signal on path 13 coupled to modulator 14. The modulator impresses analog information on the carrier, and the output of the modulator on path 15 is coupled to optical amplifier 16. The amplifier 16 amplifies the modulated signal and couples it via path 17 to utilization device 18.
In operation, analog transmitters, especially those intended for CATV applications, should have high optical output power (e.g., 50 mW) as well as high Carrier-to-Noise Ratio (CNR; e.g., 58 dB). The high output power is often obtained by optical amplification of the laser output; i.e., by means of the amplifier 16 of FIG. 2. However, optical amplification actually reduces CNR because the carrier signal from the laser beats with the amplified spontaneous emission (ASE) of the amplifier to generate well-known signal-spontaneous beat noise. In a related phenomenon, the CNR of this type of analog optical transmitter is influenced heavily by the optical input power of the amplifier, that is, the CNR decreases with decreasing input power. Therefore, the use of a laser followed by an external modulator, that usually has considerable insertion loss (e.g., as much as 6 dB), in combination with an optical amplifier, as shown in FIG. 2, can lead to unacceptable CNR values for the transmitter.
Thus, there is a need remaining in the art for an optical transmitter which delivers relatively high output powers, such as those required in CATV systems, without sacrificing CNR.